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Old 1st November 2009, 03:52 AM   #1
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Default Subwoofer speaker selector impedance question

Hey all, not sure if this is in right section,

i have recently purchased a 400w fusion car subwoofer (4 ohms) and put into a box, it will going into a car in the next 6 months,

but recently i have hooked it up to my computer,
i have a awa dvd which is the receiver, to which the subwoofer output goes to an 8 inch prolab 200w active subwoofer (i assume it is 4 ohms)

i then opened up the active sub and moved the cable that went to the speaker to a radioshack 2 way speaker selector and then ran the A channel back to the 8 inch pro lab and the B channel to the 12 inch fusion sub.

i have heard that when running both subs at the same time this increases impedance and load on the amp, but have not noticed any problems so far.

it is a passive selector being used so im not sure how i am to go about leveling the impedance to 4 ohms so that no extra load is placed on the amp,

any suggestions would be greatly appreciated.
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Old 2nd November 2009, 08:19 PM   #2
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The impedance (resistance to the flow of current) was actually reduced requiring more current for the same output voltage.

If both drivers are 4 Ohms, you now have a 2 Ohm load. If your plate amp is rated to drive a 2 Ohm load, you're fine and getting the most from your gear.

Impedance matching with solid-state amps by using a device called a transformer is not really done. A well designed matching 8:4ohm transformer would cost more than 8 plate amps and you might even have to wind one yourself as I've never seen such a thing.

summary: if the amp can't do 2 ohms, get 2 amps to drive each speaker and Y the input.
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Old 3rd November 2009, 10:30 AM   #3
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cheers mate, the amp isnt really pushed to high loads so im thinking it should be alright, but if i do choose to play music loudly i will just use one of the drivers,

also if i add a 4 ohm resistor to the input of the selctor will this help with the resistance load on the amp,

note: im not sure of the actuall minimum impedance of the amp as i cannot find any documentation on the active sub and it does not specify on the amp

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Old 3rd November 2009, 10:38 AM   #4
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Adding a resistor is pointless as you have to turn the amp up to compensate for the loss through the resistor.
__________________ my website for UK diy audio people - designs, PCBs, modules and more.
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Old 3rd November 2009, 04:27 PM   #5
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Too low of a load will cause the amp to make more heat in the general sense. A well designed amplifier will shut-off when it gets too hot before it gets damaged. Watch the heat the amp generates. If it seems too hot to the touch, it probably is too hot. Then you'll need a second amp. If it doesn't overheat, you're getting the most from your gear.
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